The present invention relates to apparatus for singularizing stacked sheets, i.e., for removing successive topmost, lowermost, foremost or rearmost sheets from a stack of flexible sheets wherein the sheets are disposed in vertical planes, in horizontal planes or in planes which are inclined with reference to a vertical as well as with reference to a horizontal plane.
It is already known to store a stack of superimposed or otherwise arrayed sheets in a magazine and to remove successive sheets of the stack, one by one, from the bottom, from the top, from the front side or from the rear side of the stack, depending on the orientation of the sheets which form the stack. Such apparatus are utilized in many types of bookbinding and like machines wherein successive folded or unfolded sheets must be delivered to a gathering or other processing station. As a rule, a singularizing apparatus comprises a flexing or bending device which can move a portion of the lowermost, foremost, uppermost or rearmost (i.e., outermost) sheet in a stack into the range of a rotary transfer conveyor. The latter comprises orbiting gripping means for releasably gripping the flexed portion of the outermost sheet and for extracting such sheet from the magazine wherein the stack is stored in response to further orbital movement of the gripping means about the axis of the transfer conveyor. Once a withdrawn sheet reaches a predetermined position, the gripping means is caused to open and to thus release the sheet which thereupon descends by gravity or is otherwise advanced toward and onto a stationary or mobile support, e.g., onto a conveyor which delivers the sheet to the next processing station. In many instances, the gripping means of the transfer conveyor comprises a gripping lever and a complementary gripping member or anvil, e.g., a cheek which can cooperate with the lever to grasp a flexed marginal portion (e.g., the folded back) of a sheet during extraction from the magazine.
As a rule, the aforementioned cheek is fixedly secured to the rotary components of the transfer conveyor which is driven at a constant peripheral speed. Therefore, when the gripping lever is caused to pinch a portion of the outermost sheet of a stack against the cheek, such sheet is abruptly accelerated from zero speed to the peripheral speed of the transfer conveyor. This is acceptable only if the peripheral speed of the transfer conveyor is relatively low and/or when the tensile strength of the material of which the sheets consist is extremely high so that the sheets can stand very pronounced stresses which develop abruptly whenever the lever begins to bias a portion of each of a series of successive outermost sheets against the cheek.
Swiss Pat. No. 374,968 discloses a singularizing apparatus wherein the speed of the transfer conveyor can be increased without necessarily damaging or destroying the sheets. The patented apparatus employs a drum which is driven by a planetary transmission and carries the cheek as well as the lever of the sheet gripping and extracting means. The planetary transmission reduces the peripheral speed of the drum to zero at the instant when the gripping lever begins to press a portion of the nearest outermost sheet against the cheek. The speed of the drum thereupon increases and such speed reaches a maximum value at the instant when the freshly extracted sheet is released, i.e., when the gripping lever moves away from the associated cheek. A drawback of the patented singularizing apparatus is that the transfer conveyor comprises a relatively large number of parts some of which are not only bulky but also heavy so that it is necessary to provide complex and expensive auxiliary equipment which compensates for the mass of the moving parts, i.e., which compensates for the fact that such heavy and bulky parts must be driven at a constantly varying speed, namely, between zero speed and a relatively high maximum speed. This contributes significantly to the initial and maintenance costs of the patented singularizing apparatus and renders the apparatus prone to malfunction. Another drawback of the patented apparatus is that, if its output is to be increased to a relatively high value which is often required in a modern gathering or like machine, the speed at which the sheets are discharged by the transfer conveyor is sufficiently high to entail at least some deformation of and/or other damage to the sheets as a result of impact against a further conveyor or against a stationary support.